1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and, more particularly, to a semiconductor integrated circuit having a current output sensor and a means for processing an output signal from the current output sensor.
2. Related Background Art
An output signal from a current output sensor is conventionally processed by a circuit having an arrangement shown in FIG. 1. Referring to FIG. 1, the current output sensor is a photodiode.
Referring to FIG. 1, a current output from a photodiode 1 must have a dynamic range of about 80 dB or more from a dark state to a bright state. When this current signal is processed by, e.g., a 5-V single power source, the signal is first logarithmically compressed by an operational amplifier 23 and a diode 33. An output Vo1 from the operational amplifier 23 is                               V          O1                =                              V            c                    -                                    kT              q                        ⁢            ln            ⁢                                          I                OP                                            I                S1                                                                        (        1        )            
where VC is the reference voltage, IS1 is the reverse saturation current of the photodiode, and IOP is the output current from the photodiode.
The output VO1 is changed into an output VO2 by a transistor 43 for compensating a dark current. The output VO2 is                               V          O2                =                              V            O1                    +                                    kT              q                        ⁢            ln            ⁢                                          I                B                                            I                S2                                                                        (        2        )            
where IB is the value of a constant current 53, and IS2 is the reverse saturation current of the transistor 43.
The output O2 is therefore                               V          O2                =                              V            C                    -                                    kT              q                        ⁢                          ln              ⁡                              (                                                                            I                      OP                                                              I                      B                                                        ·                                                            I                      S2                                                              I                      S1                                                                      )                                                                        (        3        )                                          For          ⁢                      xe2x80x83                    ⁢                      I            S1                          =                  I          S2                                    xe2x80x83                                          V          O2                =                              V            C                    -                                    kT              q                        ⁢            ln            ⁢                                          I                OP                                            I                B                                                                        (        4        )            
Referring to FIG. 1, a buffer 63 changes the impedance. An operational amplifier 93 adjusts a signal to an input range for an AD (analog-to-digital) converter in cooperation with resistors 73 and 83. An output VO from the operational amplifier 93 is                               V          O                =                              V            C                    -                                                                      R                  2                                                  R                  1                                            ·                              kT                q                                      ⁢            ln            ⁢                                          I                OP                                            I                B                                                                        (        5        )            
where R1 is the value of the resistor 73, and R2 is the value of the resistor 83. This output VO is A/D-converted and then transmitted to a CPU or the like.
In the conventional circuit, however, a plurality of circuit blocks such as an operational amplifier, constant current source, reference power source, and AD converter are required to process an output signal from the current output sensor. This increases the number of elements and the power consumption, resulting in a large chip size and high cost.
The present invention has been made in consideration of the above situation, and has as its object to process an output from a current output sensor with a smaller number of elements (simple circuit arrangement).
It is another object of the present invention to provide a semiconductor integrated circuit which can directly A/D-convert a current to achieve high-speed operation at low cost with a simpler circuit arrangement without any current-to-voltage conversion (logarithmic compression), impedance conversion, and gain adjustment.
To achieve the above objects, according to the present invention, there is provided a semiconductor integrated circuit comprising a current output sensor, and current AD conversion means connected to an output side of the current output sensor to connect an output from the current output sensor to an input side of a comparator via a current mirror circuit and A/D (analog-to-digital)-convert the output from the current output sensor.